**Preface to "New Aspects of Cancer Stem Cell Biology: Implications for Innovative Therapies"**

The cancer stem cell (CSC) paradigm represents one of the most prominent breakthroughs of the last decades in tumor biology. CSCs are broadly defined as a subpopulation of tumor cells that, besides sharing some functional features with normal stem cells (self-renewal, low cycling rate, and differentiation capacity), can drive tumor initiation, to rebuild the heterogeneity of the original tumor, to evade the immune system, and to escape conventional treatments such as chemoand radiotherapy. In light of these properties, CSCs have been implicated in tumor metastasis and recurrence. Nevertheless, the biological characteristics of CSCs, their molecular profile, their contribution to cancer progression, and even their existence, remain matters of intense debate and controversy among tumor biologists. The difficulty in achieving a better definition of CSC biology may be explained by the plasticity of such a cell subpopulation. Indeed, the emerging view is that CSCs represent a dynamic "state" of tumor cells that can acquire stemness-related properties under specific circumstances, rather than referring to a well-defined group of cells.

Regardless of their origin, it is clear that designing novel anti-tumor treatments based on the eradication of CSCs will only be possible upon unraveling the biological mechanisms that underlie their pathogenic role in tumor progression and therapy resistance. The Special Issue on "New Aspects of Cancer Stem Cell Biology: Implications for Innovative Therapies" will highlight recent insights into CSC features that can make them an attractive target for novel therapeutic strategies.

> **Ugo Cavallaro, Marco Giordano** *Editors*
